Integrated stratigraphy of the Kimmeridge Clay Formation (Upper Jurassic) based on exposures and boreholes in south Dorset, UK

For the purposes of a high-resolution multi-disciplinary study of the Upper Jurassic Kimmeridge Clay Formation, two boreholes were drilled at Swanwort h Quarry and one at Metherhills, south Dorset, UK. Together, the cores repr esent the first:complete section through the entire formation close to the type section. We present graphic logs that record the stratigraphy of the c ores, and outline the complementary geophysical and analytical data sets (g amma ray, magnetic susceptibility, total organic carbon, carbonate, delta C -13(org)). Of particular note are the new borehole data from the lowermost part of the formation which does not crop out in the type area. Detailed lo gs are available for download from the Kimmeridge Drilling Project web-site at http://kimmeridge. earth.ox.ac.uk/. Of further interest is a, mid-eudox us Zone positive shift in the delta (13)C(org)record, a feature that is als o registered in Tethyan carbonate successions, suggesting that it is a regi onal event and may therefore be useful for correlation. The lithostratigrap hy of the cores has been precisely correlated with the nearby cliff section , which has also been examined and re-described. Magnetic-susceptibility an d spectral gamma-ray measurements were made at a regular spacing through th e succession, and facilitate core-to-exposure correlation. The strata of th e exposure and core have been subdivided into four main mudrock lithologica l types: (a) medium-dark-dark-grey marl; (b) medium-dark-dark grey-greenish black shale; (c) dark-grey-olive-black laminated shale; (d) greyish-black- brownish-black mudstone. The sections also contain subordinate amounts of s iltstone, limestone and dolostone. Comparison of the type section with the cores reveals slight lithological variation and notable thickness differenc es between the coeval strata. The proximity of the boreholes:and different parts of the type section to the Purbeck-Isle of Wight Disturbance is propo sed as a likely control on the thickness changes.